May Be Superseded

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

Summary of Changes

This Last Call Working Draft provides some significant changes
since the previous version of 02 December 2008.

The definition of the value space has been changed such that it
is not necessary any more to reinterpret the value space of
xsd:string to make it a subset of the value space of
rdf:text.

The inference rules for the RDF Semantics were added.

The requirement was added that abbreviated forms must be used
in all RDF-based serialization.

Last
Call

The Working Group believes it has completed its design
work for the technologies specified this document, so this
is a "Last Call" draft. The design is not expected to
change significantly, going forward, and now is the key
time for external review, before the implementation
phase.

No Endorsement

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

1 Introduction

Many RDF [RDF] applications need a mechanism for representing text in various different languages, retrieving the text written in a specific language, and other kinds of language-specific processing. To facilitate this, RDF provides plain literals with a language tag, which form the basis for processing text in different languages in RDF. Apart from such literals, however, RDF also provides for plain literals without a language tag and typed literals. RDF thus provides three distinct types of literals each of which is treated in a separate way, which increases complexity for specifications based on RDF such as RIF and OWL. Furthermore, RDF does not provide a name for the set of all plain literals, which, for example, prevents one from stating that the range of some OWL property must be a plain literal with a language tag.

To address these deficiencies, this specification defines a datatype called rdf:text. This datatype provides a name for the set of all data values assigned to plain literals, which is why the datatype uses the rdf: prefix. Furthermore, typed rdf:text literals are semantically equivalent to plain literals, which allows specifications built on top of RDF to consider only typed literals. Since the rdf:text datatype just provides additional forms for writing plain literals, its addition does not change the semantics of RDF. Furthermore, when exchanging RDF graphs between RDF tools, typed rdf:text literals must be replaced with plain literals, thus maximizing interoperability between RDF tools that support rdf:text and those that do not.

RDF tools may use other mechanisms for representing text in different languages, such as using the xml:lang attribute on the data values of the rdf:XMLLiteral datatype. The rdf:text datatype does not provide a replacement for such mechanisms.

2 Preliminaries

A character is an atomic unit of text. Each character has a Universal Character Set (UCS) code point [ISO/IEC 10646] (or, equivalently, a Unicode code point [UNICODE]) that MUST match the Char production from XML [XML] thus ensuring compatibility with XML Schema Datatypes, version 1.1 [XML Schema Datatypes]. Code points are sometimes represented in this document as U+ followed by a four-digit hexadecimal value of the code point.

A string is a finite sequence of zero or more characters. The length of a string is the number of characters in it. Strings are written in this specification by enclosing them in double quotes. Two strings are identical if and only if they contain exactly the same characters in exactly the same sequence.

UCS [ISO/IEC 10646] and Unicode [UNICODE] provide for 1,114,112 different code points. The Char production from XML [XML], however, excludes the surrogate code points and the code points U+FFFE and U+FFFF. Thus, rdf:text provides a total of 1,112,033 different characters. This number is important, as it can affect the satisfiability of an OWL 2 ontology. Consider the following example:

This OWL 2 axiom states that the individual a:i is connected by the property a:property to at least n different strings of length one. The number of such strings is limited to 1,112,033 by the above definitions, so this ontology is satisfiable if and only if n is smaller than or equal to 1,112,033.

A language tag is a string matching the langtag production from BCP 47 [BCP 47]. Furthermore, note that this definition corresponds to the well-formed rather than the valid class of conformance in BCP 47. A language tag MAY contain subtags that are not registered in the IANA Language Subtag Registry, although an rdf:text implementation MAY also choose to reject such invalid language tags.

The language tag "en-fubar" is not registered with the IANA Language Subtag Registry, so an rdf:text implementation is allowed to reject it. This string, however, matches the langtag production from BCP 47, so it is a perfectly valid language tag for the purpose of this specification. Consequently, the value space of rdf:text (see Section 3 for its definition) contains, say, the pair 〈 "some string" , "en-fubar" 〉.

This specification uses Uniform Resource Identifiers (URIs) for naming datatypes and their components, which are defined in RFC 3986 [RFC 3986]. For readability, URIs prefixes are often abbreviated by a short prefix name according to the convention of RDF [RDF]. The following prefix names are used throughout this document:

the prefix name xs: stands for http://www.w3.org/2001/XMLSchema#

the prefix name rdf: stands for http://www.w3.org/1999/02/22-rdf-syntax-ns#

the prefix name fn: stands for http://www.w3.org/2005/xpath-functions#

the prefix name rtfn: stands for http://www.w3.org/2009/rdf-text-functions#

Datatypes are defined in this document along the lines of XML Schema Datatypes [XML Schema Datatypes]. Each datatype is identified by a URI and is described by the following components:

The value space is a set determining the set of values of the datatype. Elements of the value space are called data values.

The lexical space is a set of strings that can be used to refer to data values. Each member of the lexical space is called a lexical form, and it is mapped to a particular data value.

The facet space is a set of pairs of the form 〈 F v 〉, where F is a URI called a constraining facet, and v is an arbitrary data value called a constraining value. Each such pair is mapped to a subset of the value space of the datatype.

A plain literal is a string with an optional language tag [RDF]. A plain literal without a language tag is interpreted in an RDF interpretation by itself. A plain literal with a language tag is written as "abc"@langTag, and it is interpreted in an RDF interpretation as a pair 〈 "abc" , "langTag" 〉.

A typed literal consists of a string and a datatype URI [RDF], it is written as "abc"^^datatypeURI, and it is interpreted in an RDF interpretation as the data value that the datatype identified by datatypeURI assigns to the lexical form "abc".

The italicized keywords MUST, MUST NOT, SHOULD, SHOULD NOT, and MAY specify certain aspects of the normative behavior of tools implementing this specification, and are interpreted as specified in RFC 2119 [RFC 2119].

3 Definition of the rdf:text Datatype

The datatype identified by the URI http://www.w3.org/1999/02/22-rdf-syntax-ns#text (abbreviated rdf:text) is defined as follows.

Value Space. The value space of rdf:text consists of

all strings, and

all pairs of the form 〈 "abc" , "lc-langtag" 〉 where "abc" is a string and "lc-langtag" is a lowercase language tag.

Lexical Space. An rdf:text lexical form is a string of the form "abc@langTag" where "abc" is an arbitrary (possibly empty) string, and "langTag" is either the empty string or a (not necessarily lowercase) language tag. Each such lexical form is mapped to a data value dv as follows:

If "langTag" is empty, then dv is equal to the string "abc" and

If "langTag" is not empty, then dv is equal to the pair 〈 "abc", "lc-langtag" 〉 where "lc-langtag" is "langTag" normalized to lowercase.

The following table shows several rdf:text lexical forms and their corresponding data values.

Lexical form

Corresponding data value

"Family Guy@en"

〈 "Family Guy" , "en" 〉

"Family Guy@EN"

〈 "Family Guy" , "en" 〉

"Family Guy@FOX@en"

〈 "Family Guy@FOX" , "en" 〉

"Family Guy@"

"Family Guy"

"Family Guy@FOX@"

"Family Guy@FOX"

The following table shows several of strings that are not rdf:text lexical forms.

String

The reason for not being an rdf:text lexical form

"Family Guy"

does not contain at least one @ (U+0040) character

"Family Guy@12"

"12" is not a language tag according to BCP 47

Facet Space. The facet space of rdf:text is defined as shown in Table 1.

Table 1. The Facet Space of rdf:text

A pair 〈 F v 〉 is in the facet space of rdf:text if...

Each such pair is mapped to the subset of the value space of rdf:text containing...

...F is xs:length,xs:minLength,xs:maxLength,xs:pattern,xs:enumeration, orxs:assertions and 〈 F v 〉 is in the facet space of xs:string.

...all strings of the form "abc" and all pairs of the form 〈 "abc" , "lc-langtag" 〉 such that "abc" is contained in the subset of xs:string determined by 〈 F v 〉 as specified by XML Schema Datatypes [XML Schema Datatypes].

...F is rdf:langRange andv is an extended language range as specified in Section 2.2 of [RFC4647].

...all pairs of the form 〈 "abc" , "lc-langtag" 〉 such that "lc-langtag" matches v under extended filtering as specified in Section 3.3.2 of [RFC4647].

The facet xs:length can be used to refer to a subset of strings of a particular length regardless of whether they have a language tag or not. Thus, the subset of the value space of rdf:text corresponding to the pair 〈 xs:length 3 〉 contains the string "abc", as well as the pairs 〈 "abc" , "en" 〉 and 〈 "abc" , "de" 〉.

The facet rdf:langRange can be used to refer to a subset of strings containing the language tag. Note that the language range need not be in lowercase, and that the matching algorithm is case-insensitive. Thus, the subset of the value space of rdf:text corresponding to the pair 〈 rdf:langRange "de-DE" 〉 contains the pairs 〈 "abc" , "de-de" 〉 and 〈 "abc" , "de-de-1996" 〉 (because these match the language range "de-DE" according to RFC 4647), but not the string "abc" (because it is not a pair with a language tag) or the pairs 〈 "abc" , "de-deva" 〉 and 〈 "abc" , "de-latn-de" 〉 (because these do not match the language range "de-DE" according to RFC 4647).

The pair 〈 rdf:langRange "*" 〉 is mapped to the subset of the value space of rdf:text containing all pairs of the form 〈 "abc" , "lc-langtag" 〉. In languages such as OWL 2, this can be used to specify that a data value must contain the language tag.

4 Relationship with Plain Literals and xs:string

The definition of rdf:text has several important consequences.

The value space of rdf:text contains exactly all data values assigned to plain literals (with or without a language tag) in an RDF interpretation. Thus, the rdf:text datatype essentially just provides an explicit way of referring to this set.

The value space of rdf:text contains the value space of xs:string, as well as of all XML Schema datatypes derived from xs:string.

Typed rdf:text literals are semantically equivalent to plain literals and typed xs:string literals as shown in Table 2. Thus, in each RDF graph, one can replace a literal from the first column of Table 2 with the corresponding literal from the second column and vice versa without affecting the semantic meaning of the RDF graph.

Table 2. Correspondence between Literals

"abc@langTag"^^rdf:text

<=>

"abc"@langTag

"abc@"^^rdf:text

<=>

"abc"

"abc@"^^rdf:text

<=>

"abc"^^xs:string

In RDF implementations based on the entailment rules from Section 7 of the RDF Semantics [RDF Semantics], this equivalence can be achieved by means of the entailment rules shown in Table 3. These are analogous to rules xsd 1a and xsd 1b of the RDF Semantics [RDF Semantics] that establish semantic equivalence between typed xs:string literals and plain literals without a language tag. No rule is necessary to establish the correspondence between typed rdf:text literals and typed xs:string literals, as this is achieved indirectly via xsd 1a, xsd 1b, and the rules shown in Table 3.

Table 3. RDF Entailment Rules for rdf:text

rdft 1a

uuu aaa "abc" .

uuu aaa "abc@"^^rdf:text .

rdft 1b

uuu aaa "abc@"^^rdf:text .

uuu aaa "abc" .

rdft 2a

uuu aaa "abc"@langTag .

uuu aaa "abc@langTag"^^rdf:text .

rdft 2b

uuu aaa "abc@langTag"^^rdf:text .

uuu aaa "abc"@langTag .

Despite the semantic equivalence between typed rdf:text literals and plain literals, the presence of typed rdf:text literals in an RDF graph might cause interoperability problems between RDF tools, as not all RDF tools will support rdf:text. Therefore, before exchanging an RDF graph with other RDF tools, an RDF tool that suports rdf:textMUST replace in the graph each typed rdf:text literal with the corresponding plain literal. The notion of graph exchange includes, but is not limited to, the process of serializing an RDF graph using any (normative or nonnormative) RDF syntax.

5 Functions on rdf:text Data Values

This section defines functions that construct and operate on rdf:text data values. The terminology used and the way in which these functions are described are in accordance with the XQuery 1.0 and XPath 2.0 Functions and Operators [XPathFunc]. The error codes used in this section are given in Appendix G of the XPath 2.0 specification [XPath20] and Appendix C of XQuery and XPath function specification [XPathFunc].

5.1 Functions for Assembling and Disassembling rdf:text Data Values

5.1.1 rtfn:text-from-string

rtfn:text-from-string( $arg1 as xs:string ) as rdf:text
rtfn:text-from-string( $arg1 as xs:string, $arg2 as xs:string) as rdf:text

Summary: returns the data value 〈 $arg1, lowercase($arg2) 〉 if $arg2 is present, and returns the data value $arg1 otherwise. Both arguments must be of type xs:string or one of its subtypes, and $arg2 — if present — must be a (nonempty) language tag; otherwise, this function raises type error err:FORG0006. Note that, since the lexical forms of rdf:text require language tags to be in lowercase, this function converts $arg2 to lowercase.

5.1.2 rtfn:string-from-text

rtfn:string-from-text( $arg as rdf:text) as xs:string

Summary: returns the string part s from the argument $arg, which must be an rdf:text data value of the form 〈 s, l 〉 or of the form s. If $arg is not of type rdf:text, this function raises type error err:FORG0006.

5.1.3 rtfn:lang-from-text

rtfn:lang-from-text( $arg as rdf:text ) as xs:lang

Summary: returns the language tag l if $arg is an rdf:text data value of the form 〈 s, l 〉, and returns the empty string if $arg is an rdf:text data value of the form s. If $arg is not of type rdf:text, this function raises type error err:FORG0006.

5.2 The Comparison of rdf:text Data Values

The notion of collations used in this section is taken from Section 7.3.1 of XPath and XQuery function specification [XPathFunc].

5.2.1 rtfn:compare

rtfn:compare( $comparand1 as rdf:text?, $comparand2 as rdf:text? ) as xs:integer?

rtfn:compare( $comparand1 as rdf:text?, $comparand2 as rdf:text?, $collation as xs:string ) as xs:integer?

Summary: if either $comparand1 or $comparand2 is not of type rdf:text, of if $collation is specified but is not of type xs:string, this function raises type error err:FORG0006. Otherwise, the function returns the empty sequence if one of the arguments is empty, if one of $comparand1 and $comparand2 has a language tag and the other one does not, or if the language parts of $comparand1 and $comparand2 are unequal; otherwise, this function returns -1, 0, or 1 depending on whether the value of the string-part of $comparand1 (or $comparand1 itself, respectively, if it has no language tag) is respectively less than, equal to, or greater than the value of the string-part of $comparand2 (or $comparand2 itself, respectively, if it has no language tag). The collation used by the invocation of this function is determined according to the rules in Section 7.3.1 of the XPath and XQuery functions specification [XPathFunc].

The first version of this function backs up the XQuery operators "eq", "ne", "gt", "lt", "le", and "ge" on rdf:text values.

Feature At Risk #1: rtfn:compare

The final version of this specification might not include rtfn:compare, or it might contain an alternative solution: since xs:string values are rdf:text data values, the fn:compare function from XPath/XQuery might be extended to cover rdf:text values.

5.3 Other Functions on rdf:text Data Values

5.3.1 rtfn:length

rtfn:length($arg as rdf:text) as xs:integer

Summary: returns the number of characters in the string part s if $arg is an rdf:text data value of the form 〈 s, l 〉 or a string value s, respectively. If $arg is not of type rdf:text, this function raises type error err:FORG0006.

Feature At Risk #2: rtfn:length

The final version of this specification might not include rtfn:length, or it might contain an alternative solution: since xs:string values are rdf:text data values, the fn:string-length function from XPath/XQuery might be extended towards coverage of rdf:text values.

5.3.2 rtfn:matches-language-range

rtfn:matches-language-range($arg as rdf:text?, $range as xs:string) as xs:boolean

Summary: This function is only defined if $arg is a sequence of length 0 or 1 of type rdf:texts and $range is of type xs:string; if the parameters do not satisfy these typing conditions, the function raises a type error err:FORG0006. If the typing conditions are fulfilled, the function returns true in case $arg is an rdf:text data value of the form 〈 s, l 〉 with l a language tag that matches the extended language range $range as specified by the extended filtering algorithm for "Matching of Language Tags" [BCP-47]; otherwise, it returns false. This means that the function returns false if the argument is a string rdf:text data value. An empty input sequence is treated as a rdf:text data value consisting of the empty string, and accordingly on such input this function also returns false.

6 Acknowledgments

The RIF WG and the OWL WG made parallel efforts to support strings written in different languages. This specification is the outcome of a collaboration between the two groups, and it is based on the work on the rif:text datatype on the RIF side and the owl:internationalizedString datatype on the OWL side. A short description of the design process is available here.

ISO/IEC 10646-1:2000. Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 1: Architecture and Basic Multilingual Plane and ISO/IEC 10646-2:2001. Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 2: Supplementary Planes, as, from time to time, amended, replaced by a new edition or expanded by the addition of new parts. [Geneva]: International Organization for Standardization. ISO (International Organization for Standardization).